The furnace of the present invention can be used in any metallic coil annealing practice. Exemplary of such practices are those used in the manufacture of punching quality oriented silicon steel, regular grain oriented silicon steel and high permeability oriented silicon steel. Such silicon steels, for example, are given a high temperature final anneal at a minimum coil temperature requirement of 2150.degree. F. at soak. These temperatures are achieved in an atmosphere of pure hydrogen or a combination of hydrogen and nitrogen. It is during such an anneal that the final magnetic qualities of the silicon steel are achieved and that a mill glass (if desired) is formed on the silicon steel.
Prior art workers have devised numerous types of high temperature box annealing furnaces. Generally, such furnaces comprise a base and a removable bell. These furnaces are normally lined with refractory bricks which are tied together. Such bricks usually require periodic maintenance and replacement, at least one wall at a time. Furthermore, the refractory brick lining absorbs significant amounts of heat, lengthening the heat-up portion of the furnace cycle. Refractory bricks are also characterized by high heat retention properties which tend to prolong the cool-down portion of the furnace cycle.
Location of the heating elements of such furnaces is a matter of major concern since the manner in which the coils are heated has a direct impact on the combination of resultant physical and magnetic qualities of the product, as well as furnace productivity. Tightly wound coils of silicon steel provided with an annealing separator in the form of a magnesia coating or the like demonstrate a large difference between radial and axial heat conductivities. In general, these coils are characterized by greater heat conductivity in the axial direction of the coil, than in the radial direction. The ratio between axial and radial conductivities, depending upon the temperature range, can be as high as 20 to 1. It is also known that as the radius of a coil increases, the effective radial conductivity decreases. Axial conductivity, on the other hand, does not change with an increase in coil radius, so long as the width of the coiled silicon steel strip remains unchanged.
Ideally, heating coils from only the axial direction would be most desirable. This could be accomplished in an efficient manner with heating elements mounted on the sides of the bell if the coils to be heated were placed within the furnace with their axes horizontal. Experience has shown, however, that such an approach is unsuccessful because the coils tend to collapse under their own weight. Thus, it has been common practice to orient the one or more coils within the furnace with the eye of each coil extending vertically (i.e. with the axis of each coil vertically oriented). Prior art furnaces generally have heating elements in the base and on the roof (as well as on the side and end walls) of the bell to take advantage of heating from the axial directions.
The provision of heating elements in the base of a box annealing furnace has yielded problems which have plagued the industry for many years. When heating elements are located in the base of the furnace, it is necessary to provide a heavy steel base plate for the support of each coil together with attendant support structure for each base plate. By virtue of the heat and the weight imposed upon them, it is not uncommon for the base plates to distort or sag. This, in turn, results in localized stress within the coils mounted thereon causing distortion and yield loss. For this reason, the base plates are a constant source of maintenance problems. In addition, the base plates constitute a considerable mass to be heated and cooled, thus tending to lengthen the heating and cooling portions of the furnace cycle.
In the treatment of coils of the type contemplated by the present invention, a coil temperature of at least about 2150.degree. F. should be maintained during the soak portion of the furnace cycle. It is also important to establish a uniform temperature profile from the outer to the inner radius of a coil to minimize thermal stresses which can contribute to poor strip shape. Large temperature gradients cause the hotter portions of the coil to loosen up. This loosening of the coil convolutions exposes the entire width of the coil laps to the reducing hydrogen atmosphere, allowing the fayalite layer (formed during decarburization) to be reduced by the hydrogen. Reduction of the fayalite layer does not allow formation of a mill glass, which can be desired on grain oriented silicon steel.
The present invention is based on the discovery that in a metallic coil annealing furnace, if a ceramic base is provided (eliminating metallic coil-supporting base plates); if the bell side and end walls and roof are covered on their inside surfaces with ceramic fiber; and if the heating elements are properly located on the bell side and end walls and roof with elimination of base heating elements, this combination of elements will provide optimum product quality (both magnetic and physical), furnace productivity, low maintenance and energy savings. The use of fiber insulation in the bell yields a considerable improvement over conventional fire brick in energy consumption, furnace productivity and maintenance requirements. The use of fiber insulation reduces both the heat-up and cool-down portions of the furnace cycle. The use of a cast refractory base and the elimination of large metallic coil-supporting plates provide a number of advantages. First of all, it eliminates the costly maintenance required by the heavy steel base plates and the necessity of heating and cooling these massive plates during the furnace cycle. Secondly, the use of a cast refractory base provides a solid support structure for the entire bottom area of the coils. This distributes the coil weight uniformly and provides improved strip shape after the anneal. Thirdly, the refractory base minimizes the heat loss from the bottom of the coils. It has further been found that the combination of roof, side wall and end wall heating elements provides the maximum amount of heat to the coils without problems associated with heating elements located in the furnace base. The majority of the heating is provided by the roof elements because of the above noted heating characteristics of the coils.